Conventional train traffic control systems use physical electric blocks & require physical circuits to sense (via short circuiting action of the train wheels/axles) that it is safe for following trains to enter a section of track. When migrating over to Positive Train Control systems, needs include a reliable and highly available method to determine that the leading train has not separated (i.e. maintain train integrity). Having real-time train integrity status allows the full capacity of the train network to be better realized.
Commercially proposed methods offered include mounting a global positioning system (GPS) Receiver on the rear car of the train to monitor train speed at the rear, and monitoring train brake pipe pressure as an indirect indication that the train has not physically separated. GPS alone is not effective since sky coverage from the rear coupler of the last car on a train is very limited, and in wooded areas can be non-existent for unacceptably long periods of time. In addition, GPS visibility is variable with time of day (e.g. 5-12 satellites in an open area without nearby obstructions, depending on constellation state and user location). Typically, four satellites are required for a position solution to be computed.
Monitoring brake pipe pressure is helpful, but if an anglecock is closed somewhere along the train line, then the pressure at the rear car can remain high. Also, if the break in two occurs between cars ahead of where the anglecock was closed, air is captured in the section between the cars. That is, the telemetry data from the End of Train Device (ETD) will indicate normal air pressure is present at the end of the train, but the rear section of the train may still be separated from the head end section.